PROJECT SUMMARY/ABSTRACT Lipid peroxidation leads to the formation of oxidation specific epitopes (OSEs) that are proinflammatory and, unless they are removed, cause extensive cell damage. OSEs are present in oxidized low density lipoproteins (OxLDLs) and on the surface of apoptotic cells and cleared by binding to scavenger receptors (SRs) on macrophages. Persistence and/or excessive amounts of OSEs overwhelm the defense mechanisms against them. Additionally, the inflammatory response initiated by SR activation leads to tissue damage that becomes pathogenic in several diseases, including atherosclerosis. Natural antibodies (nAbs) produced by B1 cells of the innate immune system recognize OSEs and block their adverse effects. Epidemiologic evidence in humans as well as mechanistic studies in mice indicate that the OSE-rich OxLDLs are a pathogenic factor in both atherosclerosis and osteoporosis. Phosphocholine in oxidized phospholipids (PC-OxPL) forms OSEs that are recognized by several SRs including ScrB1 and by the nAb E06 IgM. Transgenic mice expressing a single chain (scFv) form of the antigen-binding domain of E06 IgM (E06-scFv) are protected against atherosclerosis induced by high fat diet (HFD) because the transgene prevents the binding of OSEs to macrophages and activation of inflammatory pathways. We have found that transgenic mice expressing E06-scFv have increased trabecular bone mass, accompanied by an increase in bone formation rate. The E06-scFv transgene also prevents the cortical bone loss caused by HFD by increasing endosteal osteoblast number and bone formation. E06 IgM prevents the negative effects of OxLDL on proliferation, differentiation, and survival of cultured osteoblastic cells. Our findings suggest that anti-OSE antibodies promote bone anabolism by preventing the negative effects of OSEs on osteoblasts and bone formation. Moreover, the level of anti-PC IgM (which includes E06) declines with age in mice, in association with a decline in bone mass. We hypothesize that increased production of OSEs due to accelerated apoptosis, oxidative stress, or chronic inflammation, in concert with inadequate levels of anti-OSE antibodies, contributes to the pathogenesis of osteoporosis. Studies proposed in this application will establish the cellular mechanisms of the bone anabolic effect of the E06-scFv transgene using DXA, microCT, histomorphometry, and measurement of inflammatory cytokines. Changes in the number and characteristics of osteoclast and osteoblast progenitors will be analyzed, in part by use of a novel FACS approach to study fluorescently labeled osteoprogenitors. The mechanisms by which the E06-scFv transgene attenuates HFD- induced bone loss will be further explored and the ability of this transgene to prevent the bone loss caused by old age will be determined. In addition, we will investigate whether OSEs negatively affect osteoblasts by binding to ScrB1, the most abundant SR for PC-OxPL in osteoblasts. Successful completion of this work should elucidate a previously unappreciated role of OSEs and anti-OSE antibodies in bone homeostasis and may suggest a novel therapeutic approach for the management of osteoporosis and atherosclerosis simultaneously.